1. Field of the Invention
The present invention relates to a rolled hem switching device for an overlock machine.
2. Prior Art
The overlock machine forms overlock stitches by the cooperative operations of a vertically movable needle holding a needle thread, an upper looper holding an upper looper thread passed through a thread hole at the tip end portion thereof and operates across a workpiece feed direction and a lower looper holding a lower looper thread passed therethrough. The overlock machine of this kind forms two kinds of stitches, i.e., overlock stitches formed at an edge of cloth which has been cut off by upper and lower knives and is kept in a flat state and those formed at the edge of the cut cloth which is in a rolled state, i.e., rolled hem.
Conventional overlock machines, however, have to prepare exclusive throat plates respectively dedicated to the above two kinds of stitches, being accompanied by a troublesome operation of exchanging the throat plates themselves in case of switching between the overlock stitches and the rolled hem. Moreover, in case of changing the width of the overlock stitches, the overlock machines have to further prepare some kinds of throat plates corresponding thereto.
In order to partially solve such a problem, there is a rolled hem switching device for an overlock machine disclosed in Japanese Utility Model Publication No. 5-20298. This device is provided with a fixed-pin pawl extending by the side of the needle location portion of the throat plate in the cloth feeding direction and a moving pawl arranged by the side of the fixed-pin pawl, the moving pawl being retractable.
The conventional rolled hem switching device, however, is capable of switching between forming the overlock stitches and forming the rolled hem by moving the moving pawl which can freely advance or retreat, but is incapable of adjusting the width of the overlock stitches since the moving pawl can only move forward or backward but cannot move to the left or right for adjustment due to its structure. The adjustment of width of overlock stitches involves an operation of exchanging the throat plate with another one.
Because it is a necessary condition of forming stitches of high quality for an overlock machine to properly set the distance between the needle location portion of the throat plate and the end of workpiece which has been cut off by a lower knife and a positional relationship between the end of the cut cloth and the moving pawl for catching the looper thread during sewing. If the lower knife alone is moved to change only the width of a workpiece without moving the moving pawl relative to the needle location portion of the throat plate for meeting the change of width of the overlock stitches, there occurs a problem such as rolling in the case of thin cloth, or producing a defective product having loose stitches. As a result, respective throat plates exclusively corresponding to some widths of the overlock stitches are required for forming stitches of high quality.
Moreover, it is difficult for the rolled hem switching device in the conventional overlock machine to stably hold the moving pawl which is provided under the throat plate and moves forward or backward. That is, the moving pawl mounted on the lower surface of the throat plate by a set spring which elastically presses the former against the latter from below advances forward during forming the overlock stitches so that the tip end portion of the moving pawl goes away from the position of the set spring. As a result, the moving pawl is liable to be elastically moved during forming the overlock stitches, and particularly when the moving pawl has play vertically or horizontally the tension of the needle thread or looper thread becomes unstable, resulting in a defective sewn product having uneven tension in stitches or loose stitches.